The global electronic assembly community is striving, to accommodate the replacement of Pb-containing solders, primarily Sn—Pb alloys, with Pb-free solders due to environmental regulations and market pressures. During this major transition away from eutectic or near-eutectic Sn—Pb solder (Teut=183° C.) for electronic assembly, there is also the opportunity to make a major improvement in Pb-free joint reliability for challenging operating environments, i.e., high temperatures and stress levels, as well as impact loading situations. Of the Pb-free choices, an array of solder alloys based on the Sn3.7Ag-0.9Cu (in wt. %) ternary eutectic (Teut=217° C.) composition have emerged with the most potential for broad use across the industry. U.S. Pat. No. 5,527,628 describes such Pb-free solder alloys.
The electronics industry has seized the challenge of adaptation and is proceeding rapidly to develop the assembly techniques and to generate the reliability data for tin-silver-copper (SAC) alloy solder as a favored Pb-free solder in many electronic assembly applications. Compared with Sn—Pb solders that have been limited typically to low-stress joints and reduced-temperature service because of the soft Pb phase that is prone to coarsening and ductile creep failure, the high Sn content and strong intermetallic phases of a well-designed SAC alloy solder can promote enhanced joint shear strength and creep resistance and can permit an increased operating temperature envelope for advanced electronic systems and devices.
Results of SAC alloy development have demonstrated increased shear strength at ambient temperature and elevated temperatures, e.g., 150° C. Joints made from a variety of SAC solders have also demonstrated resistance to isothermal fatigue and resistance to degradation of shear strength from thermal aging for temperature excursions up to 150° C., a current test standard for under-the-hood automotive electronics solder.
An observation that arose from initial widespread testing of SAC solder alloys was the occasional embrittlement of SAC solder joints due to micro-void nucleation, growth, and coalescence, if the exposure to elevated temperatures was sufficiently high, typically greater than about 150° C., and the exposure was sufficiently long, greater than about 500 to 1000 h (hours). This occasional joint embrittlement after thermal aging was observed at elevated Cu content in SAC solder alloys and typically was associated with excessive growth of layers of Cu-base intermetallic compounds, Cu6Sn5 and, especially, Cu3Sn. It should be noted that U.S. Pat. No. 6,231,691 provides a solder to suppress this thermal aging phenomenon through minor additions (<1 wt. %, but usually 0.2-0.3 wt. %) of a fourth element, such as Ni, Fe, and/or Co, and “like-acting elements,” to the SAC solder to suppress solid state diffusion at the solder/substrate interface that contains the Cu-base intermetallic compound layers. Later testing showed that a Mn addition was one of the most effective like-acting elemental additions, suppressing growth of both types of intermetallic layers after extensive thermal aging. This type of minor alloy addition to prevent embrittlement has become increasingly important since narrow solder joint gaps are becoming more common with miniaturization of electronic circuits.
Studies have shown that Sn dendrites are the dominant as-solidified microstructure feature in solder joints made with many SAC alloys, not a fine (ternary) eutectic, contrary to the previous experience with Sn—Pb. Also, it was found that relatively high but variable undercooling was observed commonly before joint solidification leading to Sn dendrites with spacing variations (that depend on undercooling and growth rate) but with very few distinct Sn grains. The unusually high undercooling of the SAC solder joints was associated with the difficulty of nucleating Sn solidification, as a pro-eutectic phase. Especially during slow cooling, e.g., in ball grid array (BGA) joints Where cooling rates are less than 0.2° C./s, increased undercooling of the joints also can promote formation of undesirable pro-eutectic intermetallic phases, specifically Ag3Sn “blades,” that tend to coarsen radically, leading to embrittlement of as-solidified solder joints:
References 1, 2, 3, and 4 listed below employed fourth element additions to SAC solders with the intention of avoiding Ag3Sn blades by selecting SAC compositions that were deficient in Ag and Cu, e.g., see SAC2705 [see ref. 4], SAC305, and SAC 105 [see refs. 1,2,3]. These references include the following:    1. A. W. Liu and N-C. Lee, “The Effects of Additives to Sn—Ag—Cu Alloys on Microstructure and Drop Impact Reliability of Solder Joints,” JOM, 59, no. 7 (2007) pp. 26-31    2. B. L-W. Lin et al., “Alloying modification of Sn—Ag—Cu solders by manganese and titanium,” Microelectron. Reliab. (2008), doi:10.1016/j.microrel.2008.10.001.    3. C. W. Liu, P. Bachorik, and N-C. Lee, “The Superior Drop Test Performance of SAC-Ti Solders and Its' Mechanism;” Proc 58th Electronic Components and Technology Conf, (2008), pp. 627-635.    4. D. S. K. Kang, P. A. Lauro, D.-Y. Shih, D. W. Henderson, K. J. Puttlitz, IBM J. Res. Dev. 49(4/5), 607-620 (2005).
In these references, some marginally near-eutectic SAC alloy designs were proposed with a low Cu level (0.5%) and very low Ag levels, less than 2.7% Ag [ref 4] and down to 1% Ag (SAC 105). These base alloys were selected since they would tend to promote Sn formation and inhibit nucleation of Ag3Sn [ref 1, 2, 3, 4]. Because these alloys deviate increasingly from the eutectic, they exhibit a wider melting range (mushy zone) with a liquidus temperature (for SAC 105) as high as 226° C. Unfortunately, some observations of unmodified SAC 105 interfacial failure on impact loading still occurred, since occasional high undercooling still may permit Ag3Sn blade formation during slow cooling. These “interfacial adhesion” failures prompted attempts at alloy modifications of SAC 105 solder with 1-2 additions [refs. 1,2] to improve impact resistance of BGA joints by increasing the interfacial bond strength of the intermetallic layer and, presumably, by suppressing Ag3Sn blade formation. While significant improvement in impact resistance was observed, especially for SAC105+0.13% Mn and SAC105+0.02% Ti alloys [ref. 3] (and no Ag3Sn blades were reported), their high liquidus temperature (approximately 226° C.) and wide liquid-solid mushy zone (equal to 9° C. because of the 217° C. solidus temperature) inhibits broad service application.